Attribution science measures the degree to which human-induced global warming increases the intensity or likelihood of extreme weather events such as a heatwave.Monday's UN Intergovernmental Panel on Climate Change released its first major scientific assessment since 2014. It clearly shows that global warming is happening faster than expected and that humans are almost entirely responsible.Here's a summary of the key findings of the IPCC Working Group 1 Report on Physical Science:Goodbye 1.5C, hello overshootThe Earth's average surface temperature will rise to 1.5 to 1.6 degrees Celsius above preindustrial levels by 2030, according to all five scenarios of greenhouse gas emissions. These scenarios range from extremely optimistic to recklessly considered by the report. This is a decade sooner than what the IPCC had predicted only three years ago.The 1.5C threshold will be breached by midcentury across all pathways, by about a tenth degree on the most ambitious and almost a full degree on the opposite extreme.There is a silver lining: in the most ambitious if-we-do-everything-right scenario, global temperaturesafter "overshooting" the 1.5C targetfall back to 1.4C by 2100.The natural climate is weakeningThe Earth's forests, soil, and oceans have taken 56 percent of the CO2 that humanity has released into the atmosphere since 1960. This is despite the fact that these emissions have increased by half. Earth would be much more hotter and less welcoming without the help of nature.These carbon sinks, who are allies in the fight against global warming, are showing signs of becoming saturated. The percentage of human-induced CO2 they absorb is likely to decrease as the century progresses.The ice caps are rich in enough frozen water that they can lift oceans up to 65 metres.Climate change is indeed to blameThis report highlights the remarkable progress made by attribution science in quantifying how human-induced global warming increases the intensity or likelihood of extreme weather events such as a heatwave.Scientists proved that climate change was responsible for the record-breaking heatwave in British Columbia that decimated British Columbia in June. This was just weeks after scientists had established this.The 2021 IPCC Report contains many more findings that were "highly confident" than ever before.Sea rising higher, more quicklyThe global oceans have increased by 20 centimetres (8 inches) in the past century. This has almost tripled over the last decade. The main driver of global oceans' growth has been replaced by melting glacier melt.The ocean watermark will rise by about half a metre if global warming is stopped at 2C. It will rise to almost two metres by 2300, twice the amount predicted in 2019 by the IPCC.Scientists cannot rule out two metres of total increase in emissions by 2100 due to uncertainty about the ice sheets.A selection of natural phenomena that could be dangerous to the climate if they reach critical points.Deep past warningsRecent advances in paleoclimatology, the science of Earth's natural climate in the past, have produced alarming warnings.For example, when the atmosphere of the planet was warmest, around 125,000 years ago. Global sea levels were most likely 5-10m higher. This would have put many coastal cities underwater.Three million years ago, when atmospheric carbon dioxide concentrations were comparable to today's and temperatures were 2.5C to 4C warmer, sea levels were as high as 25 metres.Methane is in the spotlightThis report contains more data than ever on methane (CH4). It is the second-most important greenhouse gas, after CO2. The report warns that failures to reduce emissions could lead to the demise of Paris Agreement goals.The human-induced sources can be divided roughly into leaks from natural gas production and coal mining, landfills, and livestock and manure processing.While CH4 does not linger in the atmosphere for as long as CO2, it is much more efficient at trapping heat. The highest levels of CH4 have been recorded in at least 800,000.Regional differences are the focusWhile all areas of the planet are warming, from the oceans to land to the air we breatheare heating at a faster rate than others. For example, in the Arctic, the average temperature on the coldest days will rise at three times the global warming rate.The sea level is rising all over the globe, but it will likely rise by up to 20% along many coasts.The sea level is rising all over the globe, but it will likely rise by up to 20% along many coasts.Tipping points = sudden changeThe IPCC warns against sudden, "low likelihood and high impact" changes in the climate system. These are known as tipping points, which can be irreversible. Examples include melting permafrost with billions of tonnes of carbon, disintegrating ice sheets that hold enough water to raise sea levels by a dozen meters and the transformation of the Amazon from tropical forest into savannah.The report states that "abrupt responses and tipping point of the climate system... cannot not be ruled out."The global ocean conveyor beltThe Atlantic Meridional Overturning Circulation, which regulates the global heat transfer from the tropics to the northern hemisphere, is slowing down. This trend is "very likely" and will continue into the 21st century.Scientists are only "medium confident" that the AMOC won't stall completely, as it did in the past. It would make European winters more severe, disrupt monsoon season, and cause sea level to rise in the North Atlantic basin.Learn more about the UN's new report on climate change2021 AFP
